An electromagnetically operating switching device, for example a contactor or a relay, whose switching contacts or main contacts switch the conductors in particular of a three-phase system, is in practice frequently subject to different wear levels on its switching contacts. This leads to failure of the switching device as soon as one of the three switching contacts which are provided in a three-phase system becomes unserviceable. This has represented a considerable restriction to the life of such switching devices, since the remaining switching contacts would often still be serviceable for some time.
This effect of different wear levels on the switching contacts, also referred to as the synchronization effect, results from the fact that the switching contacts which are subjected to wear during switching are switched at times which are not distributed in the same way statistically. One reason for this is, for example, that the switching drive via which the switching contacts are operated is driven in synchronism with the network. In this case, the switching contacts are operated at a fixed switching time with respect to that phase of the network which is used for the switching drive. Since the load on the switching contacts may be widely different at different phase angles, this leads to different wear levels of the individual switching contacts.
A method for operating an electromagnetic switching device is known from DE 41 05 698 C2. According to this document, the three phases of a three-phase network are switched during a switching operation at an advantageous time with regard to the respective phase angle of the individual currents. To do this, the method provides for the phase angle of the current to be measured in a reference phase, and for an optimized switching time to be derived from this by way of a processor. In order to achieve a uniform load on the three switching contacts, one switching drive is provided with a constant delay for switching on and off, so that the switching contacts close and open at an advantageous time. The different phase angle of the three phases is taken into account in that switching pieces of geometrically different designs are provided for operation of the switching contacts. By way of example, these have a different travel so that, during operation of the switching drive, the first phase is switched first of all, the second phase is switched after a specific delay, and the third phase is switched after a further specific delay.
Thus, according to DE 41 05 698 C2, the further phases arc switched with a delay, by way of mechanical elements, with respect to a reference phase on the basis of the determination of an optimized switching time, so that these mechanical elements also open and close at an advantageous time. However, the use of mechanical technology to set a delay time involves design complexity and its reliability is only limited.
U.S. Pat. No. 5,430,599 discloses a switching system which is intended in particular for use in high-power technology, and which takes account of temperature influences from the environment of a switching apparatus. In order to achieve a switching time which is as advantageous as possible with regard to the phase angle, a switching delay time is determined between a switching pulse and the actual opening and closing of a switching contact. The switching delay time may assume different values for the different phases in this field of high-power technology. In order to achieve a switching time which is as advantageous as possible, provision is made for each phase to be switched separately, on the basis of different switching delay times for the different phases. This has the disadvantage that an autonomous interrupter unit must be provided for each phase, and that an advantageous switching time must be determined for each phase.